This invention relates to automatic transmissions having a layshaft kinematic arrangement, particularly to such transmissions having a single input clutch but no torque converter.
Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open friction clutches and brakes, hydraulic pump, and gear meshes.
To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio, and then restoring torque. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change. This requirement can result in low quality gear ratio changes, which a vehicle operator or passenger may sense as noise, vibration and harshness.
A transmission operating in accordance with the present invention avoids entirely any interruption of power during gear ratio changes and avoids the inherent shift quality problems of ASMs. This invention, however, also reduces the magnitude of parasitic losses inherent in the operation of an automatic transmission to a much lower magnitude than is present in a conventional ASM transmission.
In order to use a launch clutch in a heavy vehicle, the transmission must provide a large torque multiplication in first gear. For layshaft transmissions, this typically results in a large center distance, the distance between the main shaft and layshaft, causing a large package size and high inertias. This is especially true with a single clutch design because the driving pinion in first gear, the smaller gear, must be journalled on its shaft. Because the pinion is journalled, its size is limited.
A transmission having a large span usually requires many gear ratios to keep the ratio steps small. Consequently, there are many gears and synchronizers or couplers in large span transmissions.